ext4: fix oops on corrupted ext4 mount From: Eric Sandeen When mounting an ext4 filesystem with corrupted s_first_data_block, things can go very wrong and oops. Because blocks_count in ext4_fill_super is a u64, and we must use do_div, the calculation of db_count is done differently than on ext4. If first_data_block is corrupted such that it is larger than ext4_blocks_count, for example, then the intermediate blocks_count value may go negative, but sign-extend to a very large value: blocks_count = (ext4_blocks_count(es) - le32_to_cpu(es->s_first_data_block) + EXT4_BLOCKS_PER_GROUP(sb) - 1); This is then assigned to s_groups_count which is an unsigned long: sbi->s_groups_count = blocks_count; This may result in a value of 0xFFFFFFFF which is then used to compute db_count: db_count = (sbi->s_groups_count + EXT4_DESC_PER_BLOCK(sb) - 1) / EXT4_DESC_PER_BLOCK(sb); and in this case db_count will wind up as 0 because the addition overflows 32 bits. This in turn causes the kmalloc for group_desc to be of 0 size: sbi->s_group_desc = kmalloc(db_count * sizeof (struct buffer_head *), GFP_KERNEL); and eventually in ext4_check_descriptors, dereferencing sbi->s_group_desc[desc_block] will result in a NULL pointer dereference. The simplest test seems to be to sanity check s_first_data_block, EXT4_BLOCKS_PER_GROUP, and ext4_blocks_count values to be sure their combination won't result in a bad intermediate value for blocks_count. We could just check for db_count == 0, but catching it at the root cause seems like it provides more info. Signed-off-by: Eric Sandeen Reviewed-by: Mingming Cao --- --- fs/ext4/super.c | 11 +++++++++++ 1 file changed, 11 insertions(+) Index: linux-2.6.24-rc3/fs/ext4/super.c =================================================================== --- linux-2.6.24-rc3.orig/fs/ext4/super.c 2007-12-05 11:41:48.000000000 -0800 +++ linux-2.6.24-rc3/fs/ext4/super.c 2007-12-05 11:41:50.000000000 -0800 @@ -1997,6 +1997,17 @@ static int ext4_fill_super (struct super if (EXT4_BLOCKS_PER_GROUP(sb) == 0) goto cantfind_ext4; + + /* ensure blocks_count calculation below doesn't sign-extend */ + if (ext4_blocks_count(es) + EXT4_BLOCKS_PER_GROUP(sb) < + le32_to_cpu(es->s_first_data_block) + 1) { + printk(KERN_WARNING "EXT4-fs: bad geometry: block count %llu, " + "first data block %u, blocks per group %lu\n", + ext4_blocks_count(es), + le32_to_cpu(es->s_first_data_block), + EXT4_BLOCKS_PER_GROUP(sb)); + goto failed_mount; + } blocks_count = (ext4_blocks_count(es) - le32_to_cpu(es->s_first_data_block) + EXT4_BLOCKS_PER_GROUP(sb) - 1);